\documentclass{article}
\usepackage{amsmath}
\begin{document}
\section{Named dots}
$A,B,\dotsc,Z$
$A + B +\dotsb +Z$
$A B +\dotsm Z$
$\int \dotsi dx$
$A B \dotso Z$
\section{Automatic dots}
$A,B,\dots,Z$
$A,B,\dots Z$
$A + B +\dots +Z$
$A + B +\dots \boldsymbol{+}Z$
$A + B +\dots -Z$
$A = B =\dots =Z$
$\int \dots dx$
$A B \dots Z$
$A B \dots$
$\frac{A \dots}{B\dots}$
\end{document}